Seepage is an important factor in the safe and long-term service of a hydraulic structure, and particularly to earth-rock and particulate structures including earth-rock dams and embankments, and so on, the seepage problem and the influence thereof are increasingly serious. It is of very important significance to ensure the engineering safety to research and develop an advanced, practical and reliable wading structure seepage detection instrument, and strengthen the rational arrangement and the efficient transmission of monitoring data and the scientific processing analysis so as to accurately identify the seepage conditions of the structure. With the rapid development of optical fiber sensing technology and the continuous development of the application fields thereof, it has become an important research topic and application direction in the optical fiber sensing technology to use the optical fiber sensing technology to detect the internal temperature changes of the wading structure and utilize the correlative mechanism of the temperature and the seepage to indirectly achieve the monitoring and identification of the seepage conditions of the structure.
A. G. S. Smekal firstly predicted from theory that after the light entered a medium, scattered light with frequency changes would appear excluding reflection and refraction. Moreover, researches show that the interaction between a photon and a phonon is processed in the form of absorbing or emitting the phonon; if a scattering phenomenon occurs when the photon absorbs or emits the phonon, the scattering of absorbing or emitting an optical phonon is called Raman scattering, and the scattering of absorbing or emitting the acoustical phonon is called Brillouin scattering, and the scattering is weakest in back-scattering light. Frequency shift occurs in both Raman scattering and Brillouin scatterings, wherein the frequency shift of Brillouin scattering is caused by sound waves or phonon waves of an acoustic branch, while the frequency shift of Raman scattering is caused by the vibration in molecules or phonon waves of an optical branch. Since the phonon describes lattice vibration, and the acoustic branch describes the motion of atomic mass centre, the frequency shift volume of Raman scattering light is unrelated to the wavelength of the incident light, and only depends on the medium properties. Further, it is considered in a quantum theory that Raman scattering is caused by the inelastic collision between the photon and a medium molecule, and the inelastic collision further leads to the transfer of energy, i.e., being represented as jumping of the molecular energy level, or absorbing the phonon and converting to scattered light with a higher frequency, or emitting the phonon and converting to scattered light with a lower frequency. When the medium molecule in a ground state jumps to an excited state through the high energy level where the incident photon is located, it will produce a Stokes photon with a lower frequency, while the medium molecule in the excited state jumps to the ground state from the high energy level after absorbing the incident photon, it produces an anti-stokes photon with a higher frequency. When being applied in a huge amount, the intensity of the anti-stokes light and the Stokes light will be continuously increased. Researches to the Raman scattering light have found that only the light intensity of the anti-stokes light is sensitive to the temperature, while neither the wavelength of the Stokes light nor the wavelength of the Raman scattering light is affected by the temperature.
According to the basic theory above, a lot of temperature and seepage measurement systems based on Raman scattering light are developed currently, but the intensity of Raman scattering light is weak, and the signal after the photoelectric conversion will be covered by a variety of noises, the signal to noise ratio is very poor, however the signal to noise ratio is often one of the most important factors determining the temperature measurement precision of the system or the distance measurement of the system. The traditional methods of increasing the signal to noise ratio include: increasing the peak power of pump pulse light, while this method has the disadvantage that when the peak power of the pulse light exceeds the nonlinear threshold of the optical fiber, a nonlinear effect will occur to Raman scattering, while the nonlinear effect will seriously interfere with the temperature demodulation; and the second method is to conduct equalization processing on the collected data repeatedly, but an overlong monitoring distance will spend much time and consume huge memory for processing, which greatly reduces the real-time reaction capacity on temperature measurement. Therefore, the spatial resolution, the length of the sensing optical fiber, the uncertainty of the temperature measurement and the measurement time become the important factors to determine the performance of the distributed optical fiber Raman temperature sensor system.
At present, the most common distributed optical fiber temperature system (Distributed Optical Fiber Temperature System, DTS) measures the temperature on the basis of the property that the Raman back-scattering light is modulated by the temperature. Since the intensity of the Raman scattering light is very weak, the DTS system is essentially a technology to process and detect weak signals, which uses the anti-stokes Raman scattering light as a temperature measurement signal, uses the single laser pulse as a pump signal, and uses the Stokes and Raman scattering light as a reference channel for temperature measurement, but has the disadvantages that the pulse width is uneasy to adjust, the spatial resolution is low, and the signal to noise ratio is poor. With the development, there are some new technologies, and those with representativeness include a distributed optical fiber temperature sensor integrated with an optical fiber Raman amplifier, a distributed optical fiber Raman temperature sensor using pulse coding technology, a distributed optical fiber temperature sensor using a Raman-related dual wavelength self-tuning technology, and a distributed optical fiber temperature sensor embedded with an optical switch.
The distributed optical fiber temperature sensor integrated with an optical fiber Raman amplifier only amplifies and increases analog electronic signals, but does not solve the problems of pulse width and signal to noise ratio fundamentally. The distributed optical fiber Raman temperature sensor using a pulse coding technology mainly aims to a single-mode optical fiber, and needs to adopt complicated coding and decoding technologies in order to improve the signal to noise ratio as well as the signal extraction and resolving ability of the system, which greatly increases the operation difficulty and the design complexity of equipment, but still has large deficiencies from the aspects of the final spatial resolution and the signal to noise ratio of the system. For the distributed optical fiber temperature sensor using a Raman-related dual wavelength self-tuning technology, using dual light sources cannot preferably ensure the same loss of the temperature measurement optical fibers of the two channels in the same band yet, and the temperature demodulation curve of the distributed optical fiber temperature sensor will still have the problems of incline, distortion, etc. For the distributed optical fiber temperature sensor embedded with an optical switch, although the temperature measurement optical fiber can be expanded into multiple channels from one channel by increasing the optical switch, the precision and measurement timeliness thereof are very difficult to ensure.
On the other hand, most of the seepage monitoring technologies based on the sensing optical fiber at current need to use an external circuit to heat the optical fiber; therefore, the optical fiber is required to have a heating function, and a set of complete heating circuit needs to be built as well, which greatly increases the manufacture cost of the optical fiber. Moreover, since it is difficult to coordinate the relationship between the voltage of the external circuit and the heating optical fiber during indoor and outdoor monitoring, the heating optical fiber generated will often have the phenomenon of instable or excessive voltage in this case, and the condition of soft fiber jacket and even charred fiber jacket will be caused in a short period of time due to the difficulty to control the voltage, which causes extremely serious damage to the operator and the instrument. In addition, when it is applied to the on-site monitoring of actual engineering, necessary security measures are often deficient, and t a heating circuit is more difficult to lay; particularly to dam and other water conservancy and hydropower engineering, since most of the engineering are located in remote regions, the service environment is extremely serious, and the achievement of the layout of optical fiber and heating function is more difficult, and even failed.
Based on the background and the current condition above, it is urgently needed to conduct major revolution and research to the optical fiber seepage measurement technology from the hydraulic seepage monitoring characteristics and special working environment, so as to provide technical support to fundamentally solve the problems of spatial resolution, signal to noise ratio, heating, and truly achieve the hydraulic seepage optical fiber monitoring with super-high spatial resolution, super-long sensing distance, super-high temperature and seepage measurement precision and super-high measurement efficiency.